The development of fully electric or hybrid vehicles has become an urgent need for sustainable long-term development.[1] The most important challenge in the near future is to find a safe, cheap and efficient battery technology that would provide electric vehicles with an extended driving range (>300 km). The corresponding increase in energy density requires the development of new chemistries for both the active electrode materials and the electrolyte.[2] Lithium metal is the ultimate anode and the only choice to complement the positive air (O2) or sulfur cathodes and to take advantage of the high specific capacities of these cathodes.[3] Nevertheless, the use of lithium metal in contact with a liquid electrolyte leads to important safety problems associated with the formation of irregular metallic lithium electrodeposits during the recharge. This would result in dendrite formation responsible for explosion hazards. To meet the requirements of the electric vehicle mass market, the Li ion batteries must improve the safety issues related to the thermal instability,[4] with formation of flammable reaction products, the possibility of leaks, and internal short-circuits. Solid-state electrolytes are the perfect solution to mitigate the lithium dendritic growth.[5] The use of a solid polymer electrolyte (SPE), where a lithium salt is associated with a polar polymer matrix, can solve most of the safety issues mentioned above. Moreover, other advantages related to the battery processing, as the lamination (Li metal, electrolyte, composite, cathode), stacking and hermetic sealing would be easier and cost-effective with a polymer electrolyte.
During the past 50 years, many polymer/lithium salt systems have been considered as replacement of liquid electrolytes in Li-ion batteries. The difficulty for the development of a suitable polymeric material resides in the ability to design a polymer that merges a high ionic conductivity and good mechanical properties.[6] The most widely studied and used systems are based in fluorinated salts dissolved in an aprotic polymer matrix of polyethylene oxide (PEO), which contains ether coordination sites that enable the dissociation of salts, together with a flexible macromolecular structure that assists ionic transport. Nevertheless, the presence of PEO crystalline regions interferes with ion transport, which requires an amorphous phase.[7] At high temperatures, above 65° C., most of the PEO based polymers become a viscous liquid and lose their dimensional stability.[8] Moreover, in the PEO-fluorinated salts systems the motion of lithium ions carries only a small fraction (⅕th) of the overall ionic current, which leads, during battery operation, to the formation of a strong concentration gradient favoring dendritic growth, which limits the power delivery.[9] For this reason, single ion polymers are preferred wherein Li+ migration is alone responsible for the ionic conduction of polymer.
In the last years, blending different types of polymers or direct copolymerization have been broadly used to match the requirements in terms of ionic conductivity and mechanical properties of SPE polymers. The advantage of a copolymer approach is the possibility of tailoring the mechanical properties as the rigidity/malleability by functionalization of the building blocks, which might include a new polymeric unit. By combining different functional units, the lithium conductivity and the electrochemical stability against alkaline metals can be improved. The mobility of the polymer chains can be enhanced by combining the copolymer with a plasticizer to avoid dense packing of the polymer and crystallization.
Up to now, however, conducting polymers that meet the mechanical and conductivity demands have not yet been provided to satisfaction. Therefore it is the goal of the present invention to provide monomers, monomer compositions and single-ion solid copolymer electrolytes with improved conductivity and/or mechanical properties.